Emphasizing Cycles Render Ambient Occlusion
I recommend you render the AO pass and do it in post-production for more control.
1. Enable the AO pass in render layers.

2. Open the node editor, enable Use nodes and do the following:


The RGB Curves node controls the contrast and the darkness of the AO, which can be helpful.
The AO pass is a Grayscale image so you need to multiply it with the render result.

Additional Explanation
In Cycles, the Ambient Occlusion option in the World tab is technically an additional homogeneous direct lighting (almost like adding an additional solid white world background lighting). However the Ambient Occlusion in Blender Internal can act as additional shading (like Multiply blending in image editor programs or in the compositor). The AO pass in Cycles is basically the shadow data only, which you can use the get the Blender Internal behaviour back, or use in other ways.